Thanks Marli,
It is certainly not the blackish Semitaspongia which Glossodoris rufomarginata has previously been found associated with. I guess the first step is to see if anyone else has found it with this sponge, as it could of course just be crawling over it, rather than eating it.

There are records on the Forum of Glossodoris sp. 14 associating with a couple of similar looking thorectid sponges, Collospongia sp, Candidaspongia sp. [see #14261, #9600], but we would obviously need to confirm the identity of the sponge and find some evidence that it actually feeds on it. It may turn out to be a dead end, but your photo certainly suggests we should consider more than one species.